1. Field of the Invention
The present invention relates to a liquid transporting apparatus which transports a liquid by applying a pressure to the liquid.
2. Description of the Related Art
Ink-jet heads having various structures have been proposed or put to practical use as ink-jet heads which transport ink to nozzles, and discharge the ink from the nozzles to a recording paper. Among these structures, an ink-jet head described in U.S. Pat. No. 6,971,738 B2 (corresponding to Japanese Patent Application Laid-open Publication No. 2004-166463) includes a channel unit (cavity plate) in which a plurality of pressure chambers communicating with nozzles is formed, and a piezoelectric actuator which applies a pressure to ink in the pressure chambers to change volume of the pressure chambers, thereby discharging ink from the nozzles.
The piezoelectric actuator of this ink-jet head includes a plurality of piezoelectric sheets which are made of a lead zirconate titanate (PZT) and are arranged to cover the pressure chambers, and individual electrodes (drive electrodes) and common electrodes which are arranged alternately between these piezoelectric sheets. The individual electrodes and common electrodes are formed in an annular form (ring form) in an area on an inner side of each of the pressure chambers along an edge thereof, as seen from a direction orthogonal to a plane of the piezoelectric sheets.
The piezoelectric actuator is structured to easily perform a so-called pulling ejection in which, after drawing ink into the pressure chamber by increasing a volume of the pressure chamber once, a substantial pressure is applied to the ink by returning the volume of the pressure chamber to its original volume. In other words, in a state that the common electrodes are kept at a ground electric potential, when a drive voltage is applied to the individual electrodes, an annular portion (ring portion) of each of the piezoelectric sheets overlapping with the edge of the pressure chamber, which is sandwiched between the individual electrode and the common electrode, is contracted in a direction parallel to a plane of the piezoelectric sheet. As a result of this, the piezoelectric sheets are deformed to project in a direction opposite to the pressure chamber. Due to the deformation of the piezoelectric sheets, the volume inside the pressure chamber is increased, and a negative pressure wave is generated inside the pressure chamber. Furthermore, when the application of the drive voltage to the individual electrodes is stopped at timing at which the pressure wave is changed to positive, the piezoelectric sheets are returned to their original shape, and the volume inside the pressure chamber is decreased as compared to the volume of the pressure chamber when the drive voltage is applied to the individual electrodes. When the volume inside the pressure chamber is decreased, the pressure wave generated with the increase in the volume of the pressure chamber and a pressure wave generated with the restoration of the piezoelectric sheet are combined, and a substantial pressure is applied to the ink. Therefore, this piezoelectric actuator can apply pressure efficiently to the ink at a comparatively low drive voltage. Further, the drive voltage is applied to the individual electrode and an electric field acts on the piezoelectric layers only at timing when the ink is discharged. Therefore, at timing other than the timing when the ink is discharged, the electric field is not applied to the piezoelectric layer, and polarization deterioration hardly occurs in the piezoelectric layers. Therefore, a durability of the actuator is also enhanced.